1. Field of the Invention
The present invention relates to a dampening water supply device in an offset printing press, and relates to a spray type dampening water supply device employing a spray nozzle. More specifically, the invention relates to a dampening water supply device which can supply dampening water in an amount exceeding a necessary amount in normal printing.
2. Description of the Related Art
In the recent years, a large number of proposals have been presented for spray type dampening water supply devices in an offset printing press for improvement of precision of a spray nozzle, no contamination of the dampening water by an ink and easiness and relatively high precision of control of a supply amount. On the other hand, as disclosed in Japanese Unexamined Patent Publication No. 52-152309, for example, it has been well known that dampening water in an amount. exceeding a necessary amount in normal printing has to be supplied in order to suppress occurrence of spoilage upon starting of a printing operation.
On the other hand, in Japanese Examined Utility Model Publication No. 2-19229, there has been disclosed a dampening water supply device which can supply the dampening water in an amount exceeding a necessary amount in the normal printing.
The dampening water supply device disclosed in the latter publication is provided with an electric-pneumatic converter performing electric-pneumatic conversion by an electric signal from speed detecting means of a rotary press to vary an air pressure in a water stage vessel connected to a spray nozzle adapting to variation of motion speed of a printing press so that supply of dampening water adapting to the motion speed of the printing press becomes possible. Furthermore, as another control current source for the electric-pneumatic converter, a constant current circuit is provided for performing electric-pneumatic conversion by a constant current for applying a desired air pressure to the water storage vessel for supplying a large amount of the dampening water upon initiation of printing operation.
On the other hand, in the printing operation, it is possible to temporarily stop the printing press in certain cases. For example, in newspaper printing, it is possible to vary a content of news, such as to include more detailed information or so forth, as time goes by, and/or to provide a page having news adapting to local delivery areas. Whenever printing of a predetermined volume is completed, the printing press is temporarily stopped to exchange the relevant plates. Particularly, in recent newspaper printing operations, plates are exchanged at high frequency in order to satisfy readers' demands by providing fresh information and information relevant to the readers as much as possible.
It has been recognized and considered to be a problem that if the dampening water is supplied in the completely same manner as at the initial starting of the printing operation whenever printing is resumed after a temporary stop for exchanging of the plate or so forth, the amount of spoilage to be caused upon resumption of the printing after the temporary stop can be greater after a greater accumulated period of printing operation before temporary stop.
While the true cause is not clear, it has been considered that according to increasing of accumulated period of the printing operation, the temperature of the ink and the plate rises and the hydrophilic property of the non-image portion is lowered due to wearing of the plate and so forth are combined reasons to make the ink to be easily deposited on the non-image portion of the plate.